Particle accelerators are machines built for the purpose of accelerating electrically charged particles to kinetic energies sufficiently high to produce certain desired nuclear reactions, ionization phenomenon, and/or materials modification processes. Typically, charged particles from an "ion source" are collimated into a "beam" and injected into accelerating structures, where they follow certain trajectories under the influence of bending, steering, focusing and accelerating fields until they have reached the required energy. At this point, the beam is typically extracted from the accelerator system and directed onto a "target", where the desired reactions occur. The by-products of these reactions can be used for scientific, medical, industrial and military applications.
Linear accelerators (linacs) are the technology of choice for the acceleration of charged particles (atomic ions) from their sources (ion sources) to the desired particle energy or to particle energies where other types of accelerators, such as synchrotrons (circular accelerators), are preferred. For protons, this often encompasses the energy range from 30 kilo-electron-volts (keV) to hundreds of million-electron-volts (MeV), or a velocity range from about 0.008 to about 0.8 times that of light.
Linacs generally involve evacuated, metallic cavities or transmission lines, filled with radio-frequency (RF) electromagnetic energy waves that result in strong alternating electric fields that can accelerate charged particles. Linac art is categorized by the properties of the RF waves, yielding two types of linacs, namely standing wave linacs and traveling wave linacs. Alternatively, linacs may be classified according to the particle velocities that they accommodate. Generally speaking, standing wave linacs are used for particle velocities less than half the speed of light (low beta linacs). Both standing wave and traveling wave linacs are used for higher velocities. At velocities close to that of light, traveling wave linacs predominate.
Common stranding wave linac structures include the relatively new radio frequency quadrupole (RFQ) linac structure, which has taken over the lowest velocity end of the linac business, the Wideroe linac, which is sometimes used for acceleration of low energy, heavy ions, the drift tube linac (DTL) structure, which holds the middle-velocities of the linac business, and the coupled cavity linac (CCL) structure, which carries the high velocity end of the standing wave linac business.
Linacs accelerate charged particles along nominally straight trajectories by means of alternating electric fields applied to linear arrays of electrodes located inside evacuated cavities. The alternating electric fields in these evacuated metallic cavities or transmission lines result from the excitation of electromagnetic cavity modes with radio frequency electromagnetic energy. Electrode spacings are arranged such that particles arrive at each gap between electrodes in an appropriate phase of the electric field to result in acceleration at each gap.
The capabilities of conventional linacs for accelerating high beam currents at low energies are severely limited by the available strengths of the conventional magnetic focusing elements, used to keep the beam diameters small enough to enable efficient interactions with the RF electric accelerating fields. In the development of linac technology, there have been numerous attempts to utilize electric :fields for the focusing forces, which, unlike magnetic fields, are independent of particle velocity and promise superior performance at the lower particle velocities. Scientists have considered both static electric quadrupole fields and time-dependent (radio frequency) electric quadrupole fields for this role.
In the early 1970's, two Russian scientists introduced the revolutionary idea of "spatially uniform strong focusing", which offers the capability of simultaneously focusing, bunching and accelerating intense beams of charged particles with RF electric fields in one compact structure, which subsequently became known as the Radio Frequency Quadrupole (RFQ) linac structure. The RFQ linacs represent the best transformation between the continuous beams that come from ion sources and the bunched beams required by most linear accelerators. Their forces, being electric, are independent of particle velocity, allowing them to focus and bunch beams at much lower energies than possible for their magnetically focused counterparts. Their capture efficiency can approach 100% with minimal emittance growth. RFQ linacs have made a major impact on the design and performance of proton, deuteron, light-ion, and heavy-ion accelerator facilities. They have set new performance standards for accelerators and in doing so have earned a role in most future proton and other ion accelerators.
However, RFQ linacs are not without limitations. In all RFQ linac structures, the acceleration rate is inversely proportional to the particle velocity. At some point in the process of particle acceleration, the acceleration rate drops to the point where some change in the acceleration process is desired. Unfortunately, in the conventional RFQ structure, there are no changes that can be made to the basic structure to rectify the inherent deterioration of the acceleration rate that occurs with higher velocities. As a result, for all but the lowest energy applications, RFQ linacs must be followed by different accelerating structures such as magnetically focused drift tube linacs (DTL), which offer higher acceleration rates in the energy ranges just beyond the practical limits of the RFQ structures up to velocities as high as half that of light. However, the magnetic focusing at this point is generally weaker than the electric focusing utilized in the RFQ structures. Consequently, matching the beam from an electrically focused RFQ linac into a magnetically focused DTL linac--often requiting several additional focusing and bunching elements and an array of beam diagnostic equipment to manage the transition--tends to be too complex and expensive for most commercial applications.
Thus, it would be valuable if this transition could be shifted to higher energies, where magnetic focusing is stronger, by either extending the energy capabilities of the RFQ structures or developing new rf-focused structures that would interface more naturally with the beams that come from RFQ linacs. In either case, new developments in linac structures would be required. However, once the beam is tightly bunched and focused in an RFQ linac, there is less need for the "spatially uniform" feature of the fields. By dropping that constraint, several avenues open up for extending the useful energy range of rf-focused linac structures.
Swenson U.S. Pat. No. 5,113,141 introduced an improved RFQ linac structure to extend the useful energy range of the conventional RFQ linac structure. The invention introduced a new degree of freedom into the system by configuring the structure as individual, four-finger-loaded acceleration/focusing cells, the orientation of which would be chosen to optimize performance. This new degree of freedom made the acceleration periodicity independent of the focusing periodicity, thus allowing the operating frequency to be raised as needed to enhance the acceleration rate without jeopardizing the required focusing action.
Another approach is to develop new linac structures based on a combination of conventional accelerating cells and four-finger-loaded focusing cells. The Russians have developed such structures to accelerate protons from 2 MeV to an energy of 30 MeV for injection into their major accelerator at Serpukov. The Russian structures involve successions of drift tubes, supported on a variety of stems, immersed in rf-cavity modes with a longitudinal magnetic field (H-modes). Half of the gaps are loaded with four fingers to supply the focusing, while the other gaps are dedicated to the acceleration. This configuration involves four or more drift tube stems per particle wavelength--a serious penalty, particularly at low particle velocities and high RF frequencies.
The present invention represents a new linac structure that combines the superior focal properties of the RFQ with the superior acceleration properties of the DTL linac. It offers strong rf-focusing and efficient RF acceleration for particles at velocities beyond that which is practical for the RFQ structure. In preferred configurations, its requires only one stem per particle wavelength.
Therefore, an object of the present invention is to provide a commercially viable linear particle accelerator suitable for interfacing with an RFQ linac to accelerate protons and heavier ions in the velocity range of about 0.05 to about 0.50 times the velocity of light.
Another object is to provide a drift tube linear accelerator excited in the TM.sub.010 RF cavity mode with RF focusing of the accelerated beam incorporated into each drift tube.
A further object is to provide a drift tube linear accelerator capable of being miniaturized.